Oscilloscopes acquire and display a record of data upon the occurrence of a particular trigger. The trigger event may occur within the signal of interest, or it may be desirable to observe a first signal while being triggered by an occurrence in a second signal. It frequently happens that a particular area of interest that the user wishes to observe, happens to be displaced in time from the trigger event. In order to observe the particular area of interest, the oscilloscope acquisition and display must be delayed by some well-defined time period. The time period is usually set by rotation of a timebase control on the front panel of the oscilloscope. In such a system, the trigger begins a time period, and acquisition and display of the waveform to be observed occurs at the expiration of the time period. This delay function is generally called holdoff and is well-known in the oscilloscope art.
A general-purpose oscilloscope is a versatile test and measurement instrument which has wide-spread application, spanning many technological fields. Notwithstanding the breadth of their application, there are some application specific functions which are commonly found on general-purpose oscilloscopes. One such set of application specific functions relate to acquisition and display of television signals.
Television signals generally comprise a plurality of television lines organized into interlaced fields. Each television line signal includes horizontal synchronizing information, color burst (color synchronizing information), and video information. Each field contains vertical synchronizing information. Two interlaced fields form a complete picture called a frame. In the NTSC television system, odd fields begin a trace at the left end of the top of the frame, and end at the center of the bottom of the frame. Even fields begin at the center of the top of the frame and end at the right end of the bottom of the frame. Thus, each field comprises 262.5 television lines. In NTSC, the fields follow a four field repeating sequence (in the PAL system, the fields follow an eight field repeating sequence). Fields 1 and 3 have the same timing, but differ from each other in that phase of their respective color burst components is offset by 180 degrees. As noted above, fields 2 and 4 differ in timing from fields 1 and 3. Fields 2 and 4 have the same timing with respect to each other, but differ from each other in that phase of their respective color burst components is offset by 180 degrees.
Many oscilloscopes have the capability of selectively triggering on just the odd (i.e., 1 and 3) or even (i.e., 2 and 4) fields. However, some oscilloscopes do not have the capability to distinguish field 1 from field 3, or field 2 from field 4. This inability to distinguish individual fields causes the oscilloscope to trigger on every odd or every even field. This unwanted dual triggering causes an overlapping display of lines from both odd or both even fields. If one is interested in observing the color burst component, what is seen is a mixture of both phases.
A common solution to this problem is to use the holdoff feature of the oscilloscope. The operator sets the trigger to ODD, but sets the holdoff to prevent triggering on the next odd field. For NTSC, the required holdoff is two fields (one frame) or 33.4 ms. For the PAL system, the holdoff is also two fields but the different vertical scan rate causes the time delay to equal 40 ms.
The holdoff resolution is a function of the timebase setting. If the timebase is set at a fast position (e.g., 1 .mu.s), then the holdoff resolution is 200 ns. Disadvantageously, it takes many turns of the holdoff control to reach the required two-field delay. Moreover, "dual standard houses" that do post production work in both NTSC and PAL would need to use a different time delay for each standard.
What is needed is an apparatus and method for quickly and easily selecting a particular television field for display.